1.10 Genetics and Behavior

Nature's blueprint: genes and heredity

Genes, segments of DNA on chromosomes, carry hereditary instructions passed from parents to offspring. Chromosomes contain thousands of genes, and humans have 23 pairs. The complete set of genetic instructions is an organism's genome. Behavioral genetics studies how genes and environment together shape traits, abilities, and disorders.

Genotype vs. phenotype

Genotype is an organism's actual genetic makeup; phenotype is the observable expression of traits, resulting from genotype interacting with environment. Most psychological traits are polygenic, influenced by many genes, not one. This is why traits like intelligence or temperament show continuous variation rather than simple either/or outcomes.

Studying nature vs. nurture

Twin studies compare identical (monozygotic) and fraternal (dizygotic) twins to estimate genetic influence on traits; higher similarity in identical twins suggests heritability. Adoption studies separate genetic and environmental effects by comparing adopted children to biological vs. adoptive parents. Heritability estimates the proportion of variation in a trait due to genetics within a population, not for a single individual.

Key terms

Gene

A segment of DNA that carries hereditary instructions, located on chromosomes.

Genotype

An organism's actual genetic makeup.

Phenotype

The observable expression of a trait, resulting from genotype and environment interacting.

Twin studies

Research comparing identical and fraternal twins to estimate the genetic contribution to traits.

Heritability

The proportion of variation in a trait within a population that is attributable to genetic differences.

Try an AP-style question

Epigenetics studies how…

1. traits skip generations
2. genes change their DNA sequence
3. hormones cause mutations
4. environment affects gene expression